As electronic systems have become more complex there has been an increasing need to verify, on an ongoing basis, that a given electronic system is in fact working properly. This testing function is often carried out by means of special diagnostic or testing hardware.
When an electronic system is first energized, and prior to the time when it starts functioning in a normal manner, it is desirable to be able to verify the operation of the testing circuitry. Experience indicates that this verification function must itself be carried out by the circuitry in question rather than by the host system. Once the testing circuitry has completed this verification sequence, it then can interact with the host system to verify the operation thereof.
In a second phase, it is also desirable to be able to continuously check the operation of the host system during its normal operation. In the event that the host system is a software controlled system, the ability to periodically respond to signals generated by the testing circuitry can readily be built into the software. In the event that the host system is a hard-wired unit, provision must be made in the hardware to generate the necessary response signals periodically.
In addition to the verification function carried out by the testing circuitry during the initial power-up phase, it is also desirable to allow the host system to itself check the status of the testing circuitry as part of an initial set of operations once the host system is permitted to start operating. This requires that the host system have the functional capability to test various conditions in the testing circuitry.
There is thus a need for hardware testing circuitry that can verify its own operation upon electrical power being applied thereto. This test circuitry should also have the functional ability to then permit the host system to begin operation. Finally, there is a continuing need for the testing circuitry to periodically generate test signals which can be sensed by the host system and responded to within a predetermined period of time. A failure of the host system within the predetermined period of time to respond to a test signal should cause the host system to halt operation.